Many clinical conditions, deficiencies, and disease states can be remedied or alleviated by supplying to the patient a one or more biologically active moieties produced by living cells or removing from the patient deleterious factors which are metabolized by living cells. In many cases, these moieties can restore or compensate for the impairment or loss of organ or tissue function. Examples of disease or deficiency states whose etiologies include loss of secretory organ or tissue function include    (a) diabetes, wherein the production of insulin by pancreatic islets of Langerhans is impaired or lost;    (b) hypoparathyroidism, wherein the loss of production of parathyroid hormone causes serum calcium levels to drop, resulting in severe muscular tetany;    (c) Parkinsonism, wherein dopamine production is diminished; and (d) anemia, which is characterized by the loss of production of red blood cells secondary to a deficiency in erythropoietin. The impairment or loss of organ or tissue function may result in the loss of additional metabolic functions. For example, in fulminant hepatic failure, liver tissue is rendered incapable of removing toxins, excreting the products of cell metabolism, and secreting essential products, such as albumin and Factor VIII. Bontempo, F. A., et al., Blood, 69, pp. 1721-1724 (1987).
In other cases, these biologically active moieties are biological response modifiers, such as lymphokines or cytokines, which enhance the patient's immune system or act as anti-inflammatory agents. These can be particularly useful in individuals with a chronic parasitic or infectious disease, and may also be useful for the treatment of certain cancers. It may also be desirable to supply trophic factors to a patient, such as nerve growth factor or insulin-like growth factor-one or -two (IGF1 or IGF2).
In still other cases, the biologically active moiety can be a secretory substance, such as a neurotransmitter, neuromodulator, hormone, trophic factor, or growth factor, or a neuroactive substance for the reduction of pain sensitivity. Such neuroactive substances include catecholamines, enkephalins, and opioid peptides.
In many disease or deficiency states, the affected organ or tissue is one which normally functions in a manner responsive to fluctuations in the levels of specific metabolites, thereby maintaining homeostasis. For example, the parathyroid gland normally modulates production of parathyroid hormone (PTH) in response to fluctuations in serum calcium. Similarly, β cells in the pancreatic islets of Lang rhans normally modulate production of insulin in response to fluctuations in serum glucose. Traditional therapeutic approaches to the treatment of such diseases cannot compensate for the responsiveness of the normal tissue to these fluctuations. For example, an accepted treatment for diabetes includes daily injections of insulin. This regimen cannot compensate for the rapid, transient fluctuations in serum glucose levels produced by, for example, strenuous exercise. Failure to provide such compensation may lead to complications of the disease state; this is particularly true in diabetes. Jarret, R. J. and Keen J., (1976) Lancet(2):1009-1012.
Many other diseases are, likewise, characterized by a deficiency in a biologically active moiety that cannot easily be supplemented by injections or longer-term, controlled release therapies. Still other diseases, while not characterized by substance deficiencies, can be treated with biologically active moieties normally made and secreted by cells. Thus, trophic and growth factors may be used to prevent neurodegenerative conditions, such as Huntington's and Alzheimer's diseases, and adrenal chromaffin cells which secrete catecholamines and enkephalins, may be used to treat pain.
It is also fairly well established that the activation of noradrenergic or opioid receptors in the spinal cord by direct intrathecal injection of α-adrenergic or opioid agonists produces antinociception, and that the co-administration of subeffective doses of these agents can produce potent analgesia. The presence of enkephalin-secreting neurons and opiat receptors in high densities in the substantia gelatinosa of the spinal cord and the resultant analgesia observed following local injection of opiates into the spinal cord have suggested a role for opioid peptides in modulating the central transmission of nociceptive information. In addition, catecholamines also appear to be important in modulating pain sensitivity in the spinal cord since injection of noradrenergic agonists into the subarachnoidal space of the spinal cord produces analgesia, while the injection of noradrenergic antagonists produces increased sensitivity to noxious stimuli.
Many drugs have been administered intraspinally in the clinical setting, and numerous methods are available to deliver intraspinal medications. For instance, the most common method of intraspinal drug delivery, particularly anesthetics, is continuous infusion by way of spinal catheters. However, the use of these catheters, particularly small-bore catheters, has been implicated in such complications as cauda equina syndrome, a neurological syndrome characterized by loss of sensation or mobility of the lower limbs. In fact, the FDA was prompted to issue a safety alert in May, 1992, alerting Anesthesia Care Providers to the serious hazard associated with continuous spinal anesthesia by small-bore catheters and has taken action to remove all small-bore catheters from the market.
Accordingly, many investigators have attempted to reconstitute organ or tissue function by transplanting whole organs, organ tissue, or cells which provide secreted products or affect metabolic functions. Moreover, transplantation can provide dramatic benefits but is limited in its application by the relatively small number of organs suitable and available for grafting. In general, the patient must be immunosuppressed in order to avert immunological rejection of the transplant, which results in loss of transplant function and eventual necrosis of the transplanted tissue or cells. In many cases, the transplant must remain functional for a long period of time, even for the remainder of the patient's lifetime. It is both undesirable and expensive to maintain a patient in an immunosuppressed state for a substantial period of time.
A desirable alternative to such transplantation procedures is the implantation of cells or tissues within a physical barrier which will allow diffusion of nutrients, waste materials, and secreted products, but block the cellular and molecular effectors of immunological rejection. A variety of devices which protect tissues or cells producing a selected product from the immune system have been explored. These include extravascular diffusion chambers, intravascular diffusion chambers, intravascular ultrafiltration chambers, and implantation of microencapsulated cells. Scharp, D. W., et al., World J. Surg., 8, pp. 221-9 (1984)2. These devices would alleviate the need to maintain the patient in an immunosuppressed state. However, none of these approaches have been satisfactory for providing long-term transplant function. A method of delivering appropriate quantities of needed substances, such as enzymes, hormones, or other factors or, providing other needed metabolic functions, for an extended period of time is still unavailable and would be very advantageous to those in need of long-term treatment.